Water 'fingerprints' show how quickly rain and snowmelt flow into rivers

New study finds a substantial portion of water in global rivers is less than three months old.

By Heath McCoy
January 2016

About one-third of the water flowing in global rivers is “young water” made up of rain and snowmelt that flowed into the river in less than three months.

This has serious implications for water pollution and ecosystem health, according to a new study published in the journal Nature Geoscience.

“The young water in the rivers is made up rain and snowfall that flowed through the landscape and into the river channel quite quickly,” says Scott Jasechko, an assistant professor with the Department of Geography at University of Calgary's Faculty of Arts.

“Calculating how long it takes for rain and melting snow to move into rivers is important, because this information helps us predict the nutrition levels in rivers and the time lags before a pollutant arrives downstream.”

Pollutants can move quickly

For the study, Jasechko worked with colleagues from universities in Saskatchewan, Switzerland and Alaska to calculate the age of water in more than 250 global rivers.

The researchers used water fingerprints known as oxygen isotopes that are found in water molecules. Just as fingerprints allow investigators to track the movement of people around crime scenes, these water fingerprints allow scientists to track water as it moves around the planet. Using this method, the research team found that much of the water in global rivers is “young,” meaning that the rain and snow flowed it into the river in less than three months.

The findings show that most landscapes may be able to move pollutants such as pesticides into stream channels rather quickly.

“Consequently, for some pollution events, there may be little time to detect the pollutant and prepare for its arrival downstream,” says Jasechko. “This is concerning considering that about two-thirds of the water that humans use is derived from surface waters.”

Flat areas, surprisingly, move water faster 

The research team also discovered that, surprisingly, flat landscapes tend to move water at a faster pace than steeper landscapes do. Generally, these flatter landscapes may generate more young stream flow because the underlying bedrock is less fractured than in steeper places. It may be harder for water to move downward past the soil in flatter landscapes, so instead of infiltrating deep underground, this water flows into the river channel.

The fact that flat areas move more young water into streams is potentially troubling, because most of the world’s croplands are found in flatter regions. Therefore, the areas where fertilizers and pesticides are most widely used are also better able to transport these agricultural products into streams relatively quickly.

Findings can help planners improve quality of water downstream

Finally, the study determines that a small amount of water stored underground generates a disproportionately large amount of the water found in global rivers. Consequently, any adverse impacts of land use, such as those brought upon by urbanization, may also be transmitted disproportionately toward downstream ecosystems.

Jasechko says, “these findings can be used to help urban and agricultural planners better assess their practices and improve the quality of water in downstream rivers and lakes.”

 

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